1. Field of the Invention
The present invention relates to a machining device. In particular, the present invention relates to a machining device capable of machining while monitoring and controlling a contact state between a workpiece and a tool.
2. Description of Related Art
Conventionally, there has been known a machining device described in Document: JP H10-217069A (on pages 4, 5, 8 and FIGS. 1, 2).
The machining device includes a machine body, a table attached to the machine body for setting a workpiece thereon, a main spindle for attaching a tool which machines the workpiece, a contact bearing interposed between the machine body and the main spindle for rotatably supporting the main spindle, a feeding electrode coaxially disposed to face the main spindle with a minute gap, and a conductor for electrically connecting the machine body with the feeding electrode. The machine body, the table, the main spindle and the feeding electrode are conductive, and also, the workpiece and the tool to be selected are conductive. Accordingly, when the workpiece and the tool come closer or contact with each other in machining, a closed-circuit is formed for connecting the workpiece, the tool, the main spindle, the feeding electrode, the conductor, the machine body, the table and again the workpiece in that order. Alternating-current is fed to the closed-circuit by an AC power supplier. The alternating-current is then detected by an electric-current detector including a resistor.
In machining, while the workpiece and the tool gradually come closer to contact with each other from their locations sufficiently spaced apart, impedance of the closed-circuit is changed along with the change in electrostatic capacitance CL of a capacitor defined by the workpiece and the tool, and then, detection current detected by the electric-current detector is changed. Owing to this, the approach and the contact of the workpiece and the tool can be sensed by way of the detection current.
With the machining device, the approach and the contact of the workpiece and the tool can be detected, however, once the workpiece contact the tool, the change in the contact state may no longer be detected. For example, when cutting is performed with a tool T contacting a workpiece surface W as shown in FIG. 10, as the tool T advances rightward in FIG. 10 and contacts a projecting portion P on the workpiece surface W, cutting load (mechanical load) between the workpiece surface W (projecting portion P) and the tool T rapidly increases. Even though there is generated the change in the above contact state, the rapid increase of the load cannot be corrected according to the above machining device described in the Document, and the cutting is forcedly performed with the excessive cutting load, thus resulting in disadvantages such as damage on the tool T and the deterioration in machining accuracy.